III. Field
The present invention relates generally to data communication, and more specifically to techniques for transmitting data in a multi-antenna communication system.
IV. Background
A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas at a transmitting entity and multiple (NR) receive antennas at a receiving entity for data transmission and is denoted as an (NT, NR) system. A MIMO channel formed by the NT transmit antennas and the NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}. The NS spatial channels may be used to transmit data in a manner to achieve greater reliability and/or higher overall throughput for the system.
The NS spatial channels of the MIMO channel may experience different channel conditions (e.g., different fading, multipath, and interference effects) and may achieve different signal-to-noise-and-interference ratios (SNRs). The SNR of a spatial channel determines its transmission capacity, which is typically quantified by a particular data rate that may be reliably transmitted on the spatial channel. For a time variant MIMO channel, the channel conditions change over time and the SNR of each spatial channel also changes over time. To maximize throughput, the MIMO system may utilize some form of feedback whereby the receiving entity evaluates the spatial channels and provides feedback information indicating the transmission capacity of each spatial channel. The transmitting entity would then adjust data transmission on the spatial channels based on the feedback information.
However, this feedback information may not be available for various reasons. For example, the MIMO system may not support transmission of feedback from the receiving entity. As another example, the MIMO channel may change more rapidly than the rate at which the receiving entity can estimate the channel and/or send the feedback information. In any case, if the transmitting entity does not know the channel conditions, then it may need to transmit data at a very low rate so that the data transmission can be reliably decoded by the receiving entity even under the worst-case channel conditions. The performance of such a system would then be dictated by the expected worst-case channel conditions.